1. Field of the Invention
The present invention relates generally to a ceramic envelope device for use in a high-pressure discharge lamp (hereinafter referred to as "HID lamp"; "HID" representing High Intensity Discharge), and in particular to a pair of closure discs in the form of electrically conducting cermet end caps which close the opposite open ends of a translucent ceramic arc tube to form a gas-tight envelope of the envelope device. More particularly, the invention is concerned with a construction for partially embedding discharge electrodes and electrical lead members in the cermet end caps.
2. Discussion of Prior Art
In the art of such HID lamps using a translucent ceramic tube, a pair of electrically conducting discs (known as end caps) are used to close the opposite open ends of the translucent ceramic tube. Examples of such closure end caps are disclosed in U.S. Pat. No. 4,155,758. Such closure discs are formed of an electrically conducting cermet prepared by mixing, for example, particles of tungsten with particles of aluminum oxide, and sintering the mixture. The electrically conducting cermet end caps support a pair of tungsten electrodes at their opposite inner surfaces defining the length of the interior of the ceramic envelope, such that the electrodes protrudes from the inner surfaces of the end caps towards each other. Additionally, power-supply lead rods or contact rods are connected or fixed to the outer surfaces of the cermet end caps by suitable methods, so that electric power is applied to the pair of opposed tungsten electrodes through the contact rods and through the cermet end caps.
Such cermet end caps have been advantageously employed, for example, in high-pressure sodium lamps, because the cermet end caps permit the use of inexpensive tungsten electrodes in place of expensive niobium electrodes. It is further recognized that the cermet end caps have been used also advantageously for so-called metal halide lamps which have translucent ceramic tubes charged with a suitable metal halide together with mercury and rare gas, because the cermet exhibits relatively high corrosion resistance to metal halides.
However, such a cermet end cap has a tendency to crack due to an excessive degree of shrinkage of its green body during a sintering process, in which an electrode and a power-supply lead member are partially embedded in the sintered cermet end cap. The green body for the cermet end cap also suffers from cracking due to a large difference in thermal expansion coefficient between the material of the cermet end cap and the materials of the electrode and lead member. Such cracks in turn cause the translucent ceramic tube to leak, thereby lowering its luminous flux and even resulting in the HID lamp failing to function.
In the light of the above inconveniences, the present inventors developed a ceramic envelope device as disclosed in co-pending patent application, Ser. No. 794,767 filed on Nov. 4, 1985, which issued as U.S. Pat. No. 4,742,269 wherein electrodes and/or lead members partially embedded in the end caps are formed of twisted metallic wires. The use of the twisted wire electrodes and/or lead members on the end caps for HID lamps has been shown effective in eliminating cracking of the conducting cermet closure discs or end caps, and thereby preventing leaking of the translucent ceramic tube of the HID lamp.
A continuing investigation by the present inventors has revealed that the arrangement employing twisted metal wires as electrodes and lead members has something to be improved. Stated more specifically, the use of a plurality of metallic strands that are twisted together into a twisted wire will cause the tip of the formed electrode, for example, to be heated to such a high temperature as to induce consumption of the electrode tip, i.e., the free end of the twisted wire. Thus, the inner wall surface of the ceramic tube tends to be easily blackened, and the luminous flux may be accordingly lowered.